1. Field of the Invention
The present invention relates to laminated piezoelectric elements and methods for manufacturing the same, and more particularly, to a laminated piezoelectric element used for manufacturing a piezoelectric actuator including a plurality of independently driven actuator units.
2. Description of the Related Art
In general, print heads mounted on inkjet-type printers are driven by piezoelectric actuators. An example of a known piezoelectric actuator is disclosed in Japanese Unexamined Patent Application Publication No. 11-320881, which includes a laminated piezoelectric element having an external shape illustrated in FIGS. 12A and 12B.
A laminated piezoelectric element 31 includes a monolithic piezoelectric body 37, i.e., a sintered ceramic compact, which includes a driver 34 having a plurality of first driving internal electrodes 32 and a plurality of second driving internal electrodes 33, the first and second driving internal electrodes being alternatively laminated with a piezoelectric layer interposed therebetween, and which includes a connector 36 having a plurality of laminated connecting internal electrodes 35, with adjacent electrodes having a piezoelectric layer interposed therebetween. Since the piezoelectric layers in the driver 34 of the monolithic piezoelectric body 37 are polarized, the piezoelectric layers expand and contract in the laminating direction X indicated in FIG. 12A, i.e., in a so-called d-33 direction thereof when an alternating voltage is applied thereto.
As shown in FIG. 12A, ends of the first driving internal electrodes 32 are exposed at one of the mutually opposing side surfaces, i.e., only at a first side surface 37a, but the other ends are not exposed at the other side surface, i.e., at a second side surface 37b of the monolithic piezoelectric body 37. Also, as shown in FIG. 12B, ends of the second driving internal electrode 33 are exposed at the second side surface 37b, but the other ends are not exposed at the first side surface 37a. Ends of the connecting internal electrodes 35 are exposed at both of the first and second side surfaces 37a and 37b. 
The monolithic piezoelectric body 37 includes a driving external electrode 38 and a connecting external electrode 39, arranged in parallel on the first side surface 37a with a spacing 40 interposed therebetween. The driving external electrode 38 and the connecting external electrode 39 are electrically connected to each of the first driving internal electrodes 32 and each of the connecting internal electrodes 35, respectively. The monolithic piezoelectric body 37 includes a common external electrode 41 provided on substantially the entire second side surface 37b and electrically connected to both the second driving internal electrodes 33 and the connecting internal electrodes 35.
Although the common external electrode 41 is provided on substantially the entire second side surface 37b, the driving external electrode 38 and the connecting external electrode 39 are provided on the first side surface 37a, each having a spacing 42 having a desired width on the bottom of the first side surface 37a, that is, each having the spacing 42 so as to be spaced away from the edge of the lower surface of the monolithic piezoelectric body 37. Thus, the driver 34 has a sectional structure illustrated in FIG. 13.
A piezoelectric actuator 45 having an external shape illustrated in FIGS. 14A and 14B is made using the laminated piezoelectric element 31. More particularly, the laminated piezoelectric element 31 is fixed on a support plate 46 with an adhesive or other suitable material, and the driver 34 includes a plurality of slits 47 formed by cutting from the upper surface toward the lower surface thereof in the laminating direction X.
As shown in FIG. 14A, the slits 47 divide each of the first and second driving internal electrodes 32 and 33, and also divide the driving external electrode 38, such that the driver 34 is divided into a plurality of portions. Thus, the divided driver 34 includes a plurality of independently driven actuator units 48 arranged therein.
In addition, a plurality of unit external electrodes 49 is provided by dividing the driving external electrode 38, wherein the unit external electrodes 49 correspond to the respective actuator units 48. The driver 34 and the connector 36 of the piezoelectric actuator 45 are divided by the slit 47, and FIG. 14B illustrates the piezoelectric actuator 45 viewed from the other side of thereof.
Furthermore, although not shown, a flexible wiring substrate associated with a drive signal source disposed outside is connected to the piezoelectric actuator 45 having the actuator units 48 provided therein. When voltages having independent polarities are applied between the corresponding unit external electrodes 49 and the common external electrode 41, that is, in practice, between the corresponding unit external electrodes 49 and the connecting external electrode 39 connected to the common external electrode 41 via the corresponding connecting internal electrodes 35, each of the actuator units 48 is driven independently of each other.
When the piezoelectric actuator 45 is made from the laminated piezoelectric element 31, the plurality of slits 47 is formed by cutting the driver 34 from the upper surface toward the lower surface of the monolithic piezoelectric body 37 in the laminating direction X so as to divide each of the first and second driving internal electrodes 32 and 33, and also divide the driving external electrode 38. In this case, the slits 47 are generally formed by making a deep cut close to the lower surface of the monolithic piezoelectric body 37 since the first and second driving internal electrodes 32 and 33 and also the driving external electrode 38 must be reliably divided.
On the other hand, since the top portion and the bottom portion of the monolithic piezoelectric body 37, which extend perpendicular to the laminating direction X, that is, the top portion and the bottom portion located above and below the first and second driving internal electrodes 32 and 33 are piezoelectrically inactive, these upper and lower portions have thicknesses that are as small as possible to further reduce the size of the laminated piezoelectric element 31. Accordingly, when the slits 47 are formed deeply, the common external electrode 41 provided on the second side surface 37b has a conducting path 41a which is sandwiched between the lower edges of the slits 47 and the lower surface of the monolithic piezoelectric body 37 and which is as narrow as, for example, about 0.1 mm to 0.2 mm.
The narrow conducting path 41a increases the electrical resistance between any two of the actuator units 48, which decreases the conductivity of the overall piezoelectric actuator 45. Further, the narrow conducting path 41a often has a disconnection because a driving current from the drive signal source flows in the conducting path 41a in a concentrated manner.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a laminated piezoelectric element in which the electrical resistance of a conducting path of a piezoelectric actuator does not increase and in which the conductivity of the piezoelectric actuator increases even when the conducting path of the common external electrode is narrow, and a method for manufacturing such a novel laminated piezoelectric element.
In accordance with a first preferred embodiment of the present invention, a laminated piezoelectric element includes a monolithic piezoelectric body having at least one slit, and a plurality of independently driven actuator units. The monolithic piezoelectric body includes a driver, and a connector. The driver includes a plurality of first driving internal electrodes, and a plurality of second driving internal electrodes, the first and second driving internal electrodes being alternatively laminated therein. The connector includes a plurality of first connecting internal electrodes laminated therein. The monolithic piezoelectric body further includes a driving external electrode, a connecting external electrode, the driving and connecting external electrodes are arranged substantially parallel to each other on one of the mutually opposing side surfaces of the monolithic piezoelectric body and electrically connected to at least the first driving internal electrodes and the first connecting internal electrodes, respectively, a common external electrode provided on the other side thereof and electrically connected to at least both the second driving internal electrodes and the first connecting internal electrodes, and a second connecting electrode electrically connected to the common external electrode. The silt extends from the upper surface toward the lower surface of the monolithic piezoelectric body in the laminating direction thereof. The actuator units are defined by dividing the driving internal electrodes and the driving external electrode with the silt. The second connecting internal electrode is provided at a bottom portion which is below the bottom edge of the slit, of the monolithic piezoelectric body, and is spaced from and substantially parallel to the driving internal electrodes.
In this configuration, the second connecting internal electrode is not divided by the slit and the second connecting internal electrode that is electrically connected to the common external electrode is provided in the bottom portion of the monolithic piezoelectric body. Thus, the electrical connection between any two of the actuator units is maintained by the second connecting internal electrode even when the width a conducting path of the common external electrode is reduced by forming the slit by cutting. Accordingly, the conductivity of the overall piezoelectric actuator does not decrease as a result of an increase in electrical resistance between any two of the actuator units and also the conducting path of the common external electrode is not broken.
In the laminated piezoelectric element, the first and second driving internal electrodes are preferably defined by printed patterns which are flat and have substantially the same shape, and the second connecting internal electrode is also defined by a printed pattern which is flat and has substantially the same shape as that of the driving internal electrodes. Further, in the laminated piezoelectric element, the second connecting internal electrode is flat and has a shape substantially the same as that of the second driving internal electrodes. That is, these configurations allow the conductivity of the piezoelectric actuator to be sufficiently and reliably maintained since the second internal electrode has a sufficient cross sectional area.
The laminated piezoelectric element preferably further includes a notch provided on a side surface of the monolithic piezoelectric body that includes the driving external electrode and the connecting external electrode provided thereon, and a conducting external electrode which is electrically connected to the second connecting internal electrode and which is provided on the side surface of the notch. The notch extends from the lower surface of the monolithic piezoelectric body upward beyond the bottom edge of the slits, but does not extend to the first and second driving internal electrodes, substantially parallel to the driving internal electrodes and the first connecting internal electrodes. In addition, one end of the second connecting internal electrode is exposed at the side surface of the notch.
With this configuration, the notch is provided along the bottom edge of the side surface, on which the driving external electrode and the connecting external electrode are provided, of the monolithic piezoelectric body, and also the conducting external electrode that is electrically connected to the second connecting internal electrode is provided on the side surface of the notch. As a result, the electrical connection between any two of the actuator units is maintained not only through the second connecting internal electrode but also through the conducting external electrode electrically connected to the second connecting internal electrode, thereby achieving further improved conductivity of the piezoelectric actuator.
In accordance with a second preferred embodiment of the present invention, a method for manufacturing the laminated piezoelectric element according to the first preferred embodiment includes the steps of supporting the monolithic piezoelectric body, having the notch provided therein, in a slanted state with respect to a depositing source or a sputtering source, and forming the driving external electrode, the connecting external electrode, and the conducting external electrode at a job lot. With this method, the conducting external electrodes are easily formed on the side surface of the notch provided in the monolithic piezoelectric body, and the conducting external electrodes are formed at the same time as the driving external electrodes and the connecting external electrodes for a plurality of the monolithic piezoelectric bodies, thereby offering an advantage in which an additional step is not needed for forming these external electrodes, that is, productivity of the laminated piezoelectric elements is greatly improved.
In accordance with a third preferred embodiment of the present invention, a piezoelectric actuator is manufactured by using the laminated piezoelectric element according to the first preferred embodiment, wherein the driver of the monolithic piezoelectric body is divided by the slit formed by cutting from the upper surface toward the lower surface of the monolithic piezoelectric body, and the plurality of independently driven actuator units is configured by dividing the first and second driving internal electrodes, which are laminated in the driver, with the slit.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.